1. Field of the Invention
The invention relates to a device for detecting properties of a web of material conveyed in longitudinal direction, e.g., a web of paper, a) with a plurality of optical waveguides having their entrance areas located each in the vicinity of the surface of the web of material and oriented to said surface and being fastened to a crossbar extending across the web of material, b) with an infrared spectrometer to which input the exit areas of the optical waveguides are connected and c) with infrared detectors at the output of the infrared spectrometer.
2. Description of the Prior Art
Devices and methods of this type are used in online process controlling in the continuous fabrication of sheet materials such as paper, textile, foils, and so on. The device permits to specifically chemically detect most of the processing chemicals. In this way, even the fastest fabrication procedures as they occur, e.g., in paper making and finishing may be monitored with sufficient accuracy. The device is therefor suited for all continuous manufacturing processes in which one or more components determine the quality. For this reason, it is mounted on mixers, calenders, padding machines, doctor blades, steamers and driers in order to be able to more constantly regulate components, additives, applications of products in coating, impregnating, laminating as well as dampness in the course of drying.
The device of the type mentioned above has been previously proposed in DE 197 09 963. In this device, the individual optical waveguides, which are also known as optical fibers and are arranged on the crossbar, are led to a switch that consecutively brings each and every single optical fiber in optical contact with a transfer fibre connected to a spectrometer for a period of time. This procedure is also known as multiplexing. Accordingly, all of the individual fibers are connected one after the other to the spectrometer for a short time period over which the optical signal detected by the corresponding individual optical fiber is interpreted.
This embodiment presents the disadvantage that on one side a mechanically operating device, viz., the switch, is used that has to provide optical contact between different optical fibers with very high accuracy. Practical use compounds the difficulty of durably operating a switch in such a manner that it accurately connects and transmits the optical signals. On the other side, the switch provides little time only to detect the signal of one single optical fiber. This signifies that the optical signal delivered by this fiber is only detected for a very small share of the overall time in time average. This makes it difficult to completely detect a web of material. It is not possible to thus obtain an overall picture of the web of material as it is increasingly demanded.
It is the object of the present invention to develop the device of the type mentioned above in such a manner that a mechanical switch, which is always complicated in manufacturing and in practical operation, is relinquished and that a higher detection rate is achieved.
Starting from the device of the type mentioned above, the solution to this object is to provide the infrared spectrometer with a holographic grating, to have the optical waveguides arranged side by side in one line at the input of the spectrometer in such a manner that the infrared spectra of the signals of the individual optical waveguides appear in rows side by side at the output of the spectrometer and that the infrared detectors are formed at the output by a detector matrix with n lines and m rows of infrared sensitive individual sensors, the spectra of up to m optical waveguides may be distributed and detected in up to n spectral areas.
This device makes it possible to concurrently monitor m areas of the web of material. The m entrance areas of the optical waveguides are oriented onto these m areas. The number of the entrance areas is preferably less than m, e.g., 0.8 times m, 0.5 times m. Of each and every single area of the web of material observed through the input range, a complete IR spectrum is permanently imaged on the matrix of the detector. It is discretional how this matrix is interpreted, but it is in any case possible to electrically detect and interpret permanently the spectra amounting to a total of up to m spectra.
Accordingly, the device according to the invention provides the possibility to virtually completely sample the web of material to be tested and examined. In other words, it becomes possible to obtain an overall picture of this web of material. As compared to the devices of the art, the detection rate is substantially higher in any event.
The detector matrices used are commercially available arrays as they are particularly utilized in cameras. Detector matrices as they may be used in the invention are offered by the firm Rockwell or by Sensors Unlimited, Inc. for example and are known as focal plane arrays. They are typically designed for the wavelength range of 0.9 to 1.7 micrometers. The number m of rows is adapted to the infrared spectrometer, the individual spectra are to substantially illuminate the individual rows. The number of rows of the detector matrix is usually higher than the number of optical waveguides, in a preferred development, at least one line is always left unused between two spectra in order to achieve a clear separation between neighboring spectra. It is moreover absolutely possible and even intentional to combine and to electrically operate in common several neighboring individual sensors and accordingly pixels. The device according to the invention utilizes detector matrices as they are in fact to be found on the market. With these matrices, the size of the individual sensors is the smallest possible so that high resolution is achieved. This however is not necessary for the device according to the invention.
Owing to the use of a holographic grating, it is possible to precisely devise the grating, the design being accurately calculated for the selected position of input and output of the spectrometer, respectively. In the preferred embodiment, the holographic grating has been given a cylindrical convex shape. In another preferred embodiment, the image of the input is created on the output by way of mirror optics. Therefore, a concave mirror is preferably arranged between input and grating, another concave mirror being preferably provided between the grating and the output.
Ratios of between 0.5 to 1 and 1 to 0.5 proved to be appropriate as an image ratio between input and output of the spectrometer, the image ratio of preference being approximately 1 to 1. Owing to the relatively small areas of the commercially available detector matrices this signifies very small surfaces for the input of the spectrometer which implies the need to use absolutely thin optical fibers at the input. Typically, optical fibers 50 to 60 micrometers in diameter are used. They are packed tightly into the input of the spectrometer, being preferably arranged in line. A zigzag line is also possible.
The use of relatively thin optical waveguides in the range of 50 to 60 micrometers at the input of the spectrometer requires that the same optical waveguides be continued until they reach the entrance area or that thicker optical waveguides, e.g., about 0.5 mm in diameter, be used at the entrance area and to then couple these to the thinner optical waveguides. The way that has been described first presents the advantage that it is not necessary to provide a coupling place between a thick and a thin optical waveguide but has the disadvantage that the extremely thin optical waveguide is very difficult to manipulate. This drawback may be addressed in that the optical waveguides are provided with a relatively thick cladding which simplifies their manipulation. This cladding only is dropped immediately in front of the input of the spectrometer where the optical waveguides are arranged in tight packing side by side. The disadvantage of the second way is that a number of up to n optical waveguides with a larger diameter has to be coupled to a same number of optical fibers with the small diameter. Although this is technically possible, it is complicated. The second way presents the advantage that it is better and easier to work with the relatively thicker optical waveguides. Both ways have comparable light efficiencies. Although the thicker optical waveguides, which are used in the second way, capture and transfer more light in the first place, this achievement is got lost at the transition from the thick to the thin optical waveguides so that in both ways each optical waveguide delivers approximately the same light flux to the spectrometer.
The lighting fixture used for examining the web of material consists as far as possible in point sources of light. Transmission or reflection may be utilized.
The interpretation of the signals of the detector matrix is carried out according to state of the art methods. Reference is made in this connection to WO 97/20429 for example. It relates to a control device for a CCD element.
The sensitivity of the individual sensors of the detector matrix varies. Methods of adjusting the sensitivity and also all the spectral properties of the individual sensors are well known and are employed for the device according to the invention. To compensate different dark tensions, a chopper may be arranged in the light path of all of the individual channels, it is preferably provided in immediate proximity to the input of the infrared spectrometer. Moreover, the detector matrix may be allocated temperature sensors so that the temperature there may be detected. Through temperature, the parameters of the detector matrix may then be compensated in as far as they depend upon the temperature. For this purpose, well-known methods such as FIR or PDS for example are employed. In this connection, reference is made to the two following publications xe2x80x9cStandardization of Near-Infrared Spectrometric Instruments, E. Bouveresse et al. Analytical Chemistry, Vol. 68, No. 6, Mar. 15, 1996xe2x80x9d and xe2x80x9cTransfer of Near-Infrared Multivariate Calibrations without Standards, Thomas B. Blank et al., Analytical Chemistry, Vo. 68, No. 17, Sep. 1, 1996.xe2x80x9d